Corey Stewart, a pro-Trump firebrand that narrowly lost the GOP race for Virginia governor, has officially announced that he is challenging Hillary Clinton’s vice-presidential candidate for his Virginia senate seat.

On Thursday, Stewart became the first GOP candidate to declare a run for Senate next year to compete against incumbent Sen. Tim Kaine, D-Va., who is running for re-election in 2018. Kaine was Hillary Clinton’s running mate in the 2016 presidential election.

Stewart, the chairman of the Prince William Board of County Supervisors, lost to Ed Gillespie for the Republican nomination in the Virginia governor race.

Known as a hardliner on immigration, Stewart served as Virginia chairman in President Trump’s campaign before he was fired after he protested without approval outside the Republican National Committee headquarters in Washington, D.C.